It is known to apply feedback control of a system with one or more on/off devices. An example of a feedback control loop 10 is shown in FIG. 1. Control loop 10 includes a conventional feedback controller 12 that produces an analog control signal u in response to a deviation of the controlled variable y from a desired set-point SP. The control signal u is applied to a switching law 14 (e.g., a pulse width modulation (PWM) controller, or the like) positioned intermediate to the feedback controller 12 and a controlled system 16. (By contrast, for a system that can be modulated, this control signal u is applied directly to a driver for the controlled system 16, which produces the desired change.) The switching law 14 responds to the control signal u by producing a pulsed output signal h (i.e., a sequence, in time, of on and off epochs) that turns the discrete devices of the controlled system 16 on and off.
The controllers for such conventional systems typically operate based on sensing a variable or parameter (i.e., a “controlled variable”) associated with the controlled devices. In these systems, however, there is often another variable “downstream” from the controlled variable (i.e., a “downstream variable”) that has variations which are more important to the desired operation of the system than the variations in the controlled variable. In situations where the time constant to effect change in the downstream variable is significantly different (e.g., larger) than the time constant for the controlled variable, the controlled variable tends to vary widely each time the devices are switched on or off, even though there is little or no change in the downstream variable. This makes it make it difficult to apply effective feedback control.
An example of such a system is a control loop for a heating, ventilation and air conditioning (HVAC) system. The HVAC system includes ventilation equipment that supplies heated or cooled air to one or more controlled spaces or target zones of a building. To maintain the controlled space at the desired temperature, the thermal output of the HVAC system must be regulated. With many HVAC systems, the ventilation equipment cannot be modulated over a continuous range but instead can only be switched to an “on” or “off” state. There are various types of known control methods that can be used to control these types of discrete systems, a well-known example being pulse width modulation (PWM).
One commonly employed HVAC system that uses such discrete devices is known as a direct expansion (“DX”) cooling system. DX cooling systems typically include a feedback controller that operates one or more compressors that can only be switched on or off. In most installations, the on/off switching of the compressors is controlled based on the temperature of the air as it comes off of the DX cooling coil (i.e., the “supply air temperature”) because it is typically not feasible to control the system by measuring temperatures in the controlled space. Based on the desired system performance, a set-point (in combination with other inputs or additional heating or cooling sources within the controlled space) is selected to provide the desired temperature of the controlled space. (For example, in a DX cooling system this set-point is typically between about 40° F. and about 65° F.—most typically about 55° F.) The supply air temperature (i.e., measured controlled variable y) is fed back to the feedback controller. The feedback controller compares the supply air temperature to the set-point and issues the control signal u to the controlled devices (e.g., turning the compressors on or off).
In such HVAC systems, the supply air temperature (i.e., the controlled variable) tends to change relatively quickly after the compressors are turned on or off. For example, when a compressor turns on, the supply air coming off the DX coil will cool rapidly; and when a compressor turns off, the air coming off the DX coil will warm rapidly. Such a quickly-reacting control loop tends to cause substantial oscillations in the controlled variable, which get fed back the controller. These wide variations or oscillations make it difficult for a feedback controller to provide stable regulation. Also, the compressors typically cannot be switched on and off too frequently. Such frequent on and off cycling is hard on the components and can lead to premature failure. A control loop having a small time constant relative to the maximum switching frequency of the components tends to make it difficult to apply feedback control.
As is well known in the HVAC field, the temperature in the controlled space (the downstream variable) is more important to the desired operation of the system than the temperature of the air coming off the cooling coil (the controlled variable). Persons located in the controlled space only care about the temperature in their immediate environment; the temperature at the cooling devices at a remote location is not relevant to anyone other than the building operators. Controlling the temperature in the controlled space is complicated by the fact that one cooling unit may serve many spaces, or the time constant of the variable being larger than the time constant for the controlled variable. This is largely due to the substantial volume of air typically found in the controlled space. As a result, the controller may be operating contrary to the desired performance of the system due to the fact that the controlled variable is insufficiently damped to reflect the true variations occurring in the downstream variable.
Although use of the downstream variable in the control scheme could be used to address the problem of insufficient damping, this downstream variable is often unavailable to the cooling device controllers in known HVAC systems. Even if the measurement were available, the existence of multiple controlled spaces and disturbances occurring between the cooling device and the controlled space would make it unreliable. Thus, it would be advantageous to provide an averaged signal (e.g., of a supply air temperature measurement) that has dynamics representative of those associated with the controlled space. It would also be advantageous to pass a supply air temperature measurement through an averaging process that has a time constant comparable to the time constant of the controlled space. It would further be desirable to provide for a control method having one or more of these or other advantageous features.